1. Field of the Invention
The present invention relates generally to a method of recording information, more particularly to a method of storing a still picture video signal and an audio signal corresponding thereto on a video disc, a video disc recorded thereon with video and audio signals and a reproducing apparatus for reproducing video and audio signals from a video disc.
2. Description of the Prior Art
As a video disc apparatus of this kind, such a video disc apparatus has hitherto been proposed in which spiral-shape recording tracks are formed on a video disc, a video signal corresponding to a still picture of one frame amount is recorded on the video disc as one circular track and an audio signal (message, background music and so on) relating to the still picture is recorded on the video disc as a plurality of circular tracks adjacent to the one circular track so that at the same time when the video signal corresponding to the still picture is reproduced to monitor the still picture, the audio information corresponding thereto can also be monitored.
The still picture can be reproduced for a desired duration by driving a video head to repeatedly trace the recording track on which the video signal corresponding to the still picture of one frame is recorded. On the other hand, in order to obtain the audio information which is sufficient for practical use, an audio signal with a period of about 10 seconds is required, while the number of the recording tracks on which the audio signal is recorded must be reduced as much as possible. To solve the above problem, it is proposed in the prior art to compress PCM (pulse code modulated) audio data in time base several tens of times to have a frequency approximately the same as that of the video signal, to convert the signal to a frequency-modulated FM signal similar to the video signal, and to record the FM signal on an audio signal track. Thus, the audio signal recorded on the audio signal track is reproduced by the same video head as that for the video signal and stored temporarily in a temporary memory circuit. Accordingly, when the stored data is read out from the temporary memory circuit while the time base thereof is being expanded so as to be converted to an audio information and the video signal is repeatedly reproduced during the period in which the stored data is converted to the audio information, the still picture and the corresponding audio information can simultaneously be reproduced.
As described above, in the conventional video disc, as shown in FIG. 1, at one side of a video signal track 1 corresponding to one still picture there are adjacently located a plurality of audio signal tracks 2 along which an audio signal corresponding to the still picture is recorded on the video signal track and at the other side of the video signal track 1 corresponding to the still picture there are also adjacently located a plurality of audio signal tracks 3 along which the audio signal corresponding to the still picture of the next adjacent video signal track is recorded. And, the video signal track 1 is traced by a video head 4 to thereby reproduce the video signal recorded on the video signal track 1. As a result, when the video signal track 1 is repeatedly reproduced by the video head 4, a cross-talk from the adjacent audio signal tracks 2 and 3 appears in the reproduced output from the video head 4 so that the quality of the reproduced still picture is inevitably deteriorated. In this connection, since the signal system of the audio signal recorded on the audio signal tracks 2 and 3 is constructed substantially the same as that of the video signal recorded on the video signal track 1, the crosstalk is apt to be caused.
In the prior art, in order to reduce the number of audio signal recording tracks as much as possible, an adaptive differential PCM (simply called ADPCM) system is adopted so as to compress and to encode the audio signal. When the audio signal is encoded according to the differential PCM system as described above, if data error occurs upon decoding, such data error is inevitably accumulated. Therefore, to reduce noise, it is desired that an error correction code having a large error correction ability is used to record the audio signal. In this connection, although the bit error ratio when digital data with a video format is recorded on the video disc is usually considered to be approximately 1.times.10.sup.-3 to 1.times.10.sup.-5, this bit error ratio is caused to be expanded.
In addition to the above requirements, in the video disc apparatus of this kind, since the audio information amount relating to each still picture is not uniform, it is necessary that the number of the audio signal tracks on the video disc can be varied in accordance with need. Further, even though the number of the audio signal tracks on the video disc is varied, digital data must be recorded on the video disc with a format so formed that upon reproducing the audio data stored in the memory circuit is conveniently read out therefrom and processed while the time base thereof is being expanded.